Human immunodeficiency virus type 1 (HIV-1) is the causative agent of acquired immunodeficiency syndrome (AIDS) and its related disorders. HIV-1 has been shown to infect and replicate in a variety of cells in vivo including CD4, T cells, certain B cells, monocytes/macrophages, Langerhans cells, and brain glial cells, and a number of virus strains have been isolated from various tissues using a variety of culture techniques. There are considerable amounts of data to suggest that viral load and immunosuppression are major determinants of the severity of HIV-1 related diseases. Moreover, it has been suggested that the level of viral load is related to the stage and progression of immunological deterioration. Thus, the measurement of viral load in patients may theoretically be useful for prognostic assessment. There are, however, no easily performed, reliable, sensitive and specific assay systems to determine the viral load at the present time. HIV-1-related antigen tests may be reliable for their specificity, but should still be considered in the developmental phase due to their limited sensitivity. For example, HIV-1 antigens such as p24 Gag protein are often undetectable in the blood of many seropositive individuals as assessed by the conventional radioimmunoassay or enzyme-linked immunosorbent assay (ELISA). The positivity of detectable circulating p24 antigen ranges from 15 to 86 % in HIV-1 antibody-positive individuals depending upon patient populations or methods employed.